Swine thyroid epithelial cells were isolated and the intracellular concentration of thyroxine was measured by radioimmunoassay and high pressure liquid chromatography. The estimated concentrations of intracellular thyroxine were 1.89 X 10(-8) micrograms/cell by radioimmunoassay, and 0.708 X 10(-8) micrograms/cell by high pressure liquid chromatography, which were equivalent to 4.39 X 10(-5) M and 1.78 X 10(-5) M, respectively. These levels were about a thousand-fold higher than that in the circulation (3.18 +/- 0.59 microgram/dl, 3.98 X 10(-8) M).